Parallel AC measurement method

ABSTRACT

A method for making electrical measurements of a first and a second DUT, the DUTs being in sufficient proximity to exhibit crosstalk therebetween, the method comprising: applying a first signal to the first DUT; applying a second signal to the second DUT, the first signal and the second signal being contemporaneous and orthogonal to each other; measuring a first DUT response; and measuring a second DUT response. The first and second DUT responses exhibit independence from the second and first signals, respectively.

BACKGROUND OF THE INVENTION

The present invention is related to making electrical measurements and,in particular, to making AC measurements on closely spaced devices.

AC measurements on devices such as semiconductor devices are importantfor reasons that include insuring that devices meet specifications andperform as expected and for the monitoring of the overall performance ofthe fabrication and/or assembly process.

Closely spaced devices are often tested separately from their neighborsto avoid crosstalk (e.g., inductive coupling, capacitive coupling, andRF coupling) that limits the available accuracy of the AC measurements.A device under test (DUT) may have an AC signal applied and the responsethereto measured. If this DUT is in close proximity of another DUT thatalso has an AC signal applied, the resulting measurement may be degradedby crosstalk from other AC signal. The degree of proximity at which thecrosstalk occurs can be a function of many parameters (e.g., frequency,power and physical structure, to name a few).

Testing DUTs separately results in the loss of the efficiency that canbe achieved with parallel (i.e., contemporaneous testing). DUTs can betested much faster in parallel. Separate testing results in highercosts, as well as increased test time.

SUMMARY OF THE INVENTION

A method for making electrical measurements of a first and a second DUT,the DUTs being in sufficient proximity to exhibit crosstalktherebetween, the method comprising: applying a first signal to thefirst DUT; applying a second signal to the second DUT, the first signaland the second signal being contemporaneous and orthogonal to eachother; measuring a first DUT response; and measuring a second DUTresponse. The first and second DUT responses exhibit independence fromthe second and first signals, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an electrical measurement system that canbe used to perform measurements according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an electrical measurement system 10 includes threesignal sources 12, 14, 16 and three measurement instruments 18, 20, 22.The sources 12, 14, 16 and the instruments 18, 20, 22 are controlled bya controller 24. The sources may be, for example, sinusoidal signalsources or sources other AC signals, including for example: squarewaves, triangle waves, pulse trains and so on. The instruments may be,for example, AC voltmeters, AC ammeters, AC power meters, frequencymeters, capacitance meters and inductance meters, including devices thatdigitize such measurements. While shown separately, the sources andinstruments (and controller) may be combined into one or morecombination apparatuses.

For testing, a signal source and a measurement instrument are attachedto respective DUTs. For example, source 12 and instrument 18 areattached to DUT 102, source 14 and instrument 20 are attached to DUT 104and source 16 and instrument 22 are attached to DUT 106.

For ease of understanding, consider the case of only two sources, twoinstruments and two DUTs, where the sources provide orthogonalsinusoidal signals. Orthogonal signals have the property that thecross-products between the signals are zero in some space of interest.

For example, to measure two DUTs using frequency f_(A), and each DUT'sresponse is observed for a time window t_(w), where t_(w) is a largeinteger multiple, M, of the period of f_(A). Stimulate the first DUTwith f_(A) and then stimulate the second DUT with

$f_{B} = {f_{A} + \frac{1}{t_{W}}}$

then the two stimuli are orthogonal. For a digitally sampled system letthe sample frequency equal N times

$\frac{1}{t_{W}}.$

Then

${f_{A} = \frac{M}{t_{W}}},{f_{s} = \frac{N}{t_{W}}},{f_{B} = \frac{M + 1}{t_{W}}}$

and the discrete Fourier transforms (DFTs) (frequency space) are

$\left. {{DUT}\mspace{14mu} A\mspace{14mu} {Real}}\Rightarrow R_{A} \right. = {\frac{1}{N}{\sum\limits_{n = 0}^{N - 1}{A_{n}*{\cos \left( {2{\pi \cdot \frac{M}{N} \cdot n}} \right)}}}}$$\left. {{DUT}\mspace{14mu} A\mspace{14mu} {Imaginary}}\Rightarrow I_{A} \right. = {\frac{1}{N}{\sum\limits_{n = 0}^{N - 1}{A_{n}*{\sin \left( {2{\pi \cdot \frac{M}{N} \cdot n}} \right)}}}}$$\left. {{DUT}\mspace{14mu} B\mspace{14mu} {Real}}\Rightarrow R_{B} \right. = {\frac{1}{N}{\sum\limits_{n = 0}^{N - 1}{B_{n}*{\cos \left( {2{\pi \cdot \frac{M + 1}{N} \cdot n}} \right)}}}}$$\left. {{DUT}\mspace{14mu} B\mspace{14mu} {Imaginary}}\Rightarrow I_{B} \right. = {\frac{1}{N}{\sum\limits_{n = 0}^{N - 1}{B_{n}*{\sin \left( {2{\pi \cdot \frac{M + 1}{N} \cdot n}} \right)}}}}$

Where A_(n) and B_(n) are the sampled responses of the DUTs bothcontaining the response of the DUT and cross talk from the other DUT.

$A_{n} = {{a\mspace{11mu} {\cos \left( {{2{\pi \cdot \frac{M}{N} \cdot n}} + \varphi_{A}} \right)}} + {b_{x}\mspace{11mu} {\cos \left( {{2{\pi \cdot \frac{M + 1}{N} \cdot n}} + \varphi_{Bx}} \right)}}}$$B_{n} = {{b\mspace{11mu} {\cos \left( {{2{\pi \cdot \frac{M + 1}{N} \cdot n}} + \varphi_{B}} \right)}} + {a_{x}\mspace{11mu} {\cos \left( {{2{\pi \cdot \frac{M}{N} \cdot n}} + \varphi_{Ax}} \right)}}}$

When these signals are applied to the DFT equations for DUT A and DUT Bthe cross talk terms, b_(x) and a_(x), sum to zero and are rejected.

The two DUT are not measured using the exact same frequency. This meansin this case, the DUTs should not have frequency dependant responses forsmall changes in measurement frequency. If M equals 1000, then M+1 isonly 0.1% higher. For many applications such a small frequencydifference will make no difference in the quality of the measurement.For this example the bandpass filters have zeros at the other frequency,thus, yield prefect rejection.

The general case allows more than two DUTs with each having its ownfrequency. For example with three DUTs the frequencies could be

${f_{A} = \frac{M - 1}{t_{W}}},{f_{B} = \frac{M}{t_{W}}},{{{and}\mspace{14mu} f_{C}} = {\frac{M + 1}{t_{W}}.}}$

In addition, there is no requirement that the DUTs be tested at nearlythe same frequency. If there are two DUTs that need to be tested each atdifferent frequency then let

${f_{A} = {{\frac{J}{t_{W}}\mspace{14mu} {and}\mspace{14mu} f_{B}} = \frac{K}{t_{W}}}},$

where both J and K are integers chosen to produce frequencies near thedesired test frequencies.

Computational operations may be performed, for example, by thecontroller, by additional controllers within the sources/instrument, orby additional computational resources associated with a still anothercontroller controlling a collection of electrical measurement systems.

The electrical measurement system 10, provides concurrent ACmeasurements of the connected DUTs without crosstalk degrading themeasurements. The measurements may then be, for example, output, stored,displayed or otherwise used.

The system 10 may be used in performing capacitance-voltage measurementsusing the method of the invention. Measurement of the C-Vcharacteristics of devices involves the measurement of capacitance withrespect to voltage. The fact that capacitance is being measured makes itimportant to minimized crosstalk between DUTs as the crosstalk is oftenitself a capacitive effect. By using the method of the invention, it ispossible to be confident that the measured values are for the DUT ofinterest, uncorrupted by the measurement of another DUT in proximity tothe one of interest.

It should be evident that this disclosure is by way of example and thatvarious changes may be made by adding, modifying or eliminating detailswithout departing from the fair scope of the teaching contained in thisdisclosure. The invention is therefore not limited to particular detailsof this disclosure except to the extent that the following claims arenecessarily so limited.

1. A method for making electrical measurements of a first and a second DUT, said DUTs being in sufficient proximity to exhibit crosstalk therebetween, said method comprising: applying a first signal to said first DUT; applying a second signal to said second DUT, said first signal and said second signal being contemporaneous and orthogonal to each other; measuring a first DUT response; and measuring a second DUT response, wherein said first and second DUT responses exhibit independence from said second and first signals, respectively.
 2. A method according to claim 1, wherein said first and second signals are sinusoids.
 3. A method according to claim 2, wherein said first and second signals are within a same bandwidth of interest.
 4. A method according to claim 2, wherein said first and second signals are in different bandwidths of interest.
 5. A method according to claim 1, further comprising applying additional orthogonal signals to respective additional DUTs and measuring respective additional DUT responses, wherein all DUT responses exhibit independence from non-respective signals.
 6. A method according to claim 1, wherein said first and second signals are voltage signals and said measured responses are indicative of capacitance. 